1. Field of the Invention
The present invention relates to a material for liquid crystal alignment that is required in alignment of liquid crystal in a process for manufacturing liquid crystal display (LCD). More particularly, the present invention relates to a norbornene monomer, a polynorbornene derivative, a liquid crystal alignment film including the same and a liquid crystal display device including the same.
2. Description of the Related Art
Since a liquid crystal display is light and small as compared to a known Braun tube, the liquid crystal display is used in notebook computers or navigator systems of vehicles. Currently, in accordance with the rapid increase of demand for large monitors, advantages of lightweight and slimness of the liquid crystal display are raised, such that the use of the liquid crystal display in desktop computers is increased.
The most liquid crystal display is a thin film transistor liquid crystal display (TFT-LCD), and light that is emitted from a fluorescent lamp is incident into a liquid crystal panel by reflection and dispersion devices. This light may be blocked by a liquid crystal layer in which birefringence is changed by voltage controlled by the thin film transistor (TFT) or may pass through them.
The liquid crystal panel forms a lower substrate of a cell by layering a thin film transistor driving device and an ITO (Indium-Tin-Oxide) transparent electrode on a glass substrate and then layering the alignment film. On the internal surfaces of a pair of upper and lower substrates, a spacer is formed by a sealant in order to inject the liquid crystal material therebetween, a polarizing film is attached to the external surfaces of the glass substrates, and finally, the liquid crystal material is injected between a pair of substrates and cured to manufacture the liquid crystal display cell.
In this TFT-LCD, in order to use the liquid crystal as an optical switch, it is required that the liquid crystal disposed on the layer on which the thin film transistor is formed at the innermost portion of the display cell is initially aligned in a predetermined direction. In order to implement this, the liquid crystal alignment film is used.
As the method for aligning the liquid crystal, currently, in most thin film transistor liquid crystal displays, a rubbing method in which liquid crystal is aligned by physically contacting a substrate on which a polymer is coated using soft cotton or nylon velvet is widely used.
However, the alignment of the liquid crystal by the rubbing has problems in that the thin film transistor may be broken or cross track short may occur because of the electrostatics generated by the rubbing, and defects may be increased by attracting circumferential dust.
Accordingly, in order to solve the problems of the rubbing method, the non-contact type of liquid crystal alignment methods are studied, and the method having the highest possibility among them is an alignment method for manufacturing a liquid crystal alignment film by using light irradiation.
The possibility of the light alignment method was found by using an azobenzene compound (K. Ichimura et al., Langmuir, 4, 1214, 1988). Thereafter, various kinds of polymer compounds such as polymaleimide (H. J. Choi et al., U.S. Pat. No. 6,218,501), polyolefine (R. H. Herr et al., U.S. Pat. No. 6,201,087) and the like have been developed as a photoalignment material.
Among them, in the case of polymer that includes the cinnamate-based photoreactive group at a side chain thereof, the anisotropic property is formed by the cyclobutane ring that is formed through the [2+2] cycloaddition reaction, such that the excellent liquid crystal alignment can be obtained. However, in general, since the photoreactivity is low and the thermal stability is poor, the direction of generated cyclobutane is little changed, and thus the anisotropic property is changed. Accordingly, there is a problem in that the reliabilities of the alignment film and the liquid crystal display device that adopts the alignment film are lowered.